The present invention relates to vibrators for handling large loads, and more particularly, to vibrator controllers that are synchronized to a subharmonic of an electrical power line frequency.
Vibratory material handling of large loads requires the use of long strokes applied at a low rate or low frequency. These frequencies are significantly lower than those capable of delivery by conventional electromagnetic gap motors. Prior art vibrators include mechanical exciters utilizing eccentric weight(s) secured to a shaft rotated by an electric motor or other prime mover. The bearings rotatably mounting the shaft, and the motor armature when directly coupled to the shaft, are subjected to high loads resulting from the oscillating forces created by the rotation of the eccentric weight(s), which loads adversely affect the service life of the bearings. In addition, the environment in which the vibratory mechanism must operate is often dusty, or otherwise not conductive to long bearing life, resulting in wear of shaft and/or bearings and aggravating the service problems relating to these bearings. Because of the heat generated by the motor, and the fact that its output motion is rotational, shrouding of the motor is difficult and/or very costly. Consequently, the use of such mechanical exciters is generally restricted to environments where exposed electric motors do not represent a hazard.
The electromagnetic vibrators, which do not employ a rotating mass, but instead reciprocate a mass in a straight line motion, have no bearings, obviating the problems associated therwith, and can be completely enclosed or shrouded. However, such vibrators require a controller to supply electric power to an electromagnetic, or preferably dual opposing magnets which are alternately energized. In order to maximize available stroke amplitudes for the reciprocating mass, static forces acting thereon must be eliminated to the extent possible. This can be achieved by both energizing the magnets for an electrical conduction period which is of equal time duration and magnitude for each magnet, and synchronizing the magnets to turn on in exactly opposite phases to each other.